MAC Flashcards
MAC
minimum alveolar concentration
the lowest concentration of an anesthetic agent at which 50% of patients show no response to painful stimulus
what is the relationship between MAC and potency of an anesthetic agent?
agents with low MAC are more potent
what are the calculations for determining what percentage of an anesthetic agent to run?
1 x MAC = light anesthesia
1.5 x MAC = moderate anesthesia
2 x MAC = deep anesthesia
what factors does MAC vary with?
age, body temperature, metabolic activity
disease, gestation, obesity can alter potency
how do we usually run inhalant anesthesia compared to the value of MAC?
we usually run it higher than MAC
moderate anesthesia plane is ideal
relationship between solubility and MAC
higher solubility = higher potency = lower MAC
lower solubility = lower potency = higher MAC
what is the goal of using sedatives, opioids, muscle relaxers, etc along with anesthesia?
to lower MAC
we want moderate anesthesia plane with less inhalant anesthesia
partition coefficient
solubility is expressed as partition coefficient which is the ratio of the concentration of an agent in 2 substances
blood-gas partition coefficient
the ratio of the concentration of an inhalant agent in the blood and in the alveolar gas
blood-gas partition coefficient relationship between solubility and blood/gas
poorly soluble = decreased blood/gas
highly soluble = increased blood/gas
effect of low blood-gas partition coefficient on solubility
BGPC of 0.5 (low) indicates that anesthetic is 1/2 as soluble in the blood as it is in the alveolar gas
means that 2/3 of anesthetic is in the alveolar gas and 1/3 of it is in blood
ideal because we want more anesthetic in lungs so they wake up faster after
effect of high blood-gas partition coefficient on solubility
BGPC of 2 (high) indicates anesthetic is twice as soluble in the blood as in the alveolar gas
means that 1/3 of anesthetic is in alveolar gas and 2/3 is in blood (hanging out in tissues)
what does partition coefficient indicate?
it indicates the speed of induction, change of depth, and recovery expected for agent
lower BGPC = faster induction, change of depth, and recovery
high partition coefficient relationship with solubility
high PC is highly soluble in blood and tissues
readily absorbed into blood and tissues
high levels don’t build up in alveolus = low concentration gradient - slow diffusion
high PC slow to leave tissues = slow recovery
vapor pressure
amount of pressure exerted by gaseous form of a substance when the gas and liquid states are in equilibrium
equilibrium: # of molecules leaving liquid = # of molecules reentering
measure of an inhalant’s tendency to evaporate
what is vapor pressure dependent on?
anesthetic agent and temperature
how is high vapor pressure described as?
volatile
volatile agents must be delivered from precision vaporizer
what vaporizer do you use with low vapor pressure agents?
they don’t require a precision vaporizer
halogenated anesthetics
liquids at room temperature and turn into gas with O2
stored in vaporizers
fresh gas
O2 + anesthetic
halogenated anesthetics mode of action
inhibits nerve cell function in the brain and spinal cord, inhibits neurotransmitter-gated ion channels and gamma-aminobutyric acid (GABA) receptors, diffuses across alveolar cell membranes to blood stream
how is the rate of diffusion of halogenated anesthetics controlled?
controlled by the concentration gradient between alveolus, bloodstream, and lipid solubility of drug
effect of increased lipophilicity
increased lipophilicity –> increased ability to cross neuronal membranes –> increased potency
how is anesthesia maintained?
with sufficient quantities of the agent in the blood, alveoli, and brain
decreased quantity administered —> decreased alveolar concentration —> reverses concentration gradience
effects of halogenated anesthetics
CNS: dose related reversible depression
CV: depression, vasodilation, decreased cardiac output
resp: depress vasodilation (Vt and RR)
adverse effects of halogenated anesthetics
CNS: increased ICP
CV: hypotension —> decreased renal blood flow
resp: hypoventilation —> increased CO2 —> respiratory acidosis
isoflurane
most common inhalant anesthetic
approved for dogs and horses
high vapor pressure: 240 mmHg
vaporizer setting range: 0.25-5%
low blood-gas partition coefficient: 1.46
MAC ~1.3-1.6%
good for renal disease patients
mostly excreted through lungs and < 1% is metabolized/excreted through liver/kidneys
low PC so change in depth is quick
sevoflurane
2nd most common inhalant anesthetic
used in dogs
vapor pressure: 160 mmHg
precision vaporizer required
blood-gas partition coefficient: 0.68
non-irritating and odorless
vaporizer setting range: 0.25-8%
MAC ~2.3-2.58%
less potent than iso = higher concentration needed
mainly excreted through lungs and 2-5% excreted through liver/kidneys
desflurane
newest inhalant anesthetic
lowest blood-gas partition coefficient: 0.42
“1 breath anesthesia”
vapor pressure: 700 mmHg
boiling point: 74.3 F
used with a special electronic heated vaporizer
MAC ~7.2-9.8%
vaporizer setting range: 1-18%
mainly excreted through lungs and < 1% excreted through liver/kidneys
